Method of making solid electrolytic capacitor having oxidized anode wire

ABSTRACT

A solid electrolyte capacitor includes a capacitor element, an anode lead, and a cathode lead. The capacitor element includes a capacitor chip, an anode wire projecting from the capacitor chip, and a cathode electrode formed on outer surfaces of the capacitor chip. The anode lead is electrically connected to the anode wire, whereas the cathode lead is electrically connected to the cathode electrode. A method for making such a solid electrolyte capacitor includes a laser irradiation step for irradiating the anode wire with a laser beam, and a connection step for connecting the anode wire with the anode lead after the laser irradiation step.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method of making a capacitor,in particular a solid electrolyte capacitor. The present invention alsorelates to a method of processing a capacitor element incorporated insuch a solid electrolyte capacitor.

[0003] 2. Description of the Related Art

[0004]FIG. 14 shows a known solid electrolyte capacitor (see e.g. U.S.Pat. No. 5,693,104) The solid electrolyte capacitor shown in FIG. 14comprises a capacitor element 90 enclosed in a resin package 91. Theresin package 91 also encloses part of external connection leads 92, 93.The remaining parts of the leads 92, 93 extend out of the resin package91. One lead 92 (cathode lead) is connected to a cathode electrode 90 aformed on the capacitor element 90, whereas the other lead 93 (anodelead) is connected to an anode wire. 90 b extending out of the capacitorelement 90. The anode lead 93 may be connected to the anode wire 90 b bymeans of resistance welding or thermocompression bonding for example.

[0005] The capacitor element 90 is provided by forming a dielectriclayer and a solid electrolyte layer in pores of a sintered porous mass,and then forming the cathode electrode 90 a. The anode wire 90 b ispartially enclosed in the sintered porous mass. The anode wire 90 b maybe buried in the sintered porous mass before forming the solidelectrolyte layer.

[0006] By burying the anode wire 90 b in the sintered porous mass beforethe solid electrolyte layer is formed, however, the surface of the anodewire 90 b are oxidized due to the heating treatment included in theforming process of the solid electrolyte layer. Further, the sinteredporous mass is formed by sintering a compressed metal powder. If theanode wire 90 b is buried before this sintering process, it becomesoxidized during that process. In any case, to connect the anode wire 90b and the anode lead 93, an oxidized film exists therebetween.Consequently, the oxidized film inhibits proper bonding (performed bycompatibility, atomic diffusion or alloying) between the metal in theanode wire 90 b and the metal in the anode lead 93 even if energy isapplied to the connecting portion. The oxidized film may remain, for allthe applied energy, at the boundary surface (interfacial alloyed layer)and weaken the connection between the anode wire 90 b and the anode lead93.

[0007] In order to eliminate the above problem, it has been proposed toremove the oxidized film on the anode wire by sandblasting or partiallyremove the oxidized film by making a cut in the anode wire. However, itbecomes more difficult to perform these mechanical treatments as thedownsizing of the capacitor element proceeds. Further, these mechanicaltreatments give a large load on the anode wire as well as the portion inthe sintered mass where the anode wire is buried, which may lead tobreakage of the anode wire or ill-conduction between the anode wire andthe sintered mass (the main part of the element), thereby deterioratingelectric characteristics. In particular, the anode wire becomes thinnerin accordance with the downsizing of the capacitor element, whereby theill conduction or the breakage of the anode wire is more likely tooccur.

SUMMARY OF THE INVENTION

[0008] It is, therefore, an object of the present invention to provide amethod of making a capacitor, particularly a solid electrolyte capacitorwhile eliminating or at least reducing the above-described problems.

[0009] Another object of the present invention is to provide a method ofprocessing a capacitor element incorporated in such a capacitor withadvantages.

[0010] According to a first aspect of the present invention, there isprovided a method for making a capacitor that comprises: a capacitorelement including a first and a second electrodes; a first leadelectrically connected to the first electrode; and a second leadelectrically connected to the second electrode. The method comprises: alaser irradiation step for irradiating the first electrode with a laserbeam; and a connection step for connecting the first electrode to thefirst lead after the laser irradiation step.

[0011] In a preferred embodiment, the first electrode may comprise ametal wire on which an oxidized film is formed. The laser irradiation inthe laser irradiation step may be continued until at least a part of theoxidized film is removed and a metal surface of the wire is exposed.

[0012] Preferably, in the connection step, the exposed metal surface ofthe wire may be held in contact with the first lead, and energy isapplied to the contact portion.

[0013] Preferably, in the laser irradiation step, selective removalmaybe performed exclusively with respect to a portion on the wire thatcomes into contact with the first lead.

[0014] Preferably, the connection step may employ resistance welding.

[0015] According to a second aspect of the present invention, there isprovided a method for making a solid electrolyte capacitor. Thecapacitor comprises: a capacitor element including an element chip, ananode wire projecting from the element chip, and a cathode electrodeformed on outer surfaces of the element chip; an anode lead electricallyconnected to the anode wire; a cathode lead electrically connected tothe cathode electrode; and a resin package enclosing the capacitorelement, a portion of the anode lead, and a portion of the cathode lead.The method of making the capacitor comprises: a laser irradiation stepfor irradiating the anode wire with a laser beam; and a connection stepfor connecting the anode wire to the anode lead after the laserirradiation step.

[0016] In a preferred embodiment, the anode wire may be formed with anoxidized film. In the laser irradiation step, the laser irradiation maybe continued until at least a part of the oxidized film is removed and ametal surface of the wire is exposed.

[0017] Preferably, in the connection step, the exposed metal surface ofthe anode wire may be held in contact with the anode lead, and energymay be applied to the contact portion.

[0018] Preferably, in the laser irradiation step, selective removal maybe performed exclusively with respect to a portion on the wire thatcomes into contact with the first lead.

[0019] Preferably, the connection step may employ resistance welding.

[0020] According to a third aspect of the present invention, there isprovided a method of processing a capacitor element comprising: anelement chip; an anode wire projecting from the element chip andincluding a surface formed with an oxidized film; and a cathodeelectrode formed on an outer surface of the element chip. The method maycomprise a step of: removing at least a part of the oxidized film on theanode wire by laser irradiation.

[0021] Other objects, features, and advantages invention will becomeclearer from the description of the embodiment given below withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a sectional view showing a capacitor obtained by afabrication method according to the present invention.

[0023]FIG. 2 is a sectional view showing a capacitor element of thecapacitor in FIG. 1.

[0024]FIG. 3 is a view illustrating a laser irradiation step.

[0025]FIG. 4a is a perspective view showing the entity of the capacitorelement after the laser irradiation.

[0026]FIG. 4b is a sectional view taken along IVb-IVb in FIG. 4a.

[0027]FIG. 5a is a perspective view showing a principal portion ofanother capacitor element after the laser irradiation.

[0028]FIG. 5b is a sectional view taken along Vb-Vb in FIG. 5a.

[0029]FIG. 6 is a plan view showing a principal portion of a lead frameused in the method of making according to the present invention.

[0030]FIG. 7 is a sectional view taken along VIII-VIII in FIG. 6.

[0031]FIG. 8 is a sectional view of a principal portion illustrating amounting step of the capacitor element.

[0032]FIG. 9 is a sectional view of a principal portion illustrating aconnection step of an anode wire.

[0033]FIG. 10 is a sectional view taken along XI-XI in FIG. 9.

[0034]FIG. 11 is a sectional view of a principal portion illustrating aresin packaging step where molds are utilized.

[0035]FIG. 12 is a sectional view showing the capacitor immediatelyafter taken out of the molds.

[0036]FIG. 13 is a graph showing results of an experiment.

[0037]FIG. 14 is a sectional view showing a capacitor obtained by aconventional method of making.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] Preferred embodiments of the present invention will be describedbelow with reference to the accompanying drawings.

[0039] Referring to FIG. 1, a capacitor X comprises a capacitor element1 and leads 20, 21 for external connection. The capacitor element 1 isenclosed in a resin package 3. The external connection leads 20, 21 aredistinguished as needed hereinafter by being referred to as “cathodelead” and “anode lead”, respectively.

[0040] The leads 20, 21, respectively, include inner lead portions 20 a,21 a enclosed in the resin package 3 as well as outer lead portions 20b, 21 b extending out of the resin package 3. The outer lead portions 20b, 21 b are cranked so that their ends extend horizontally on a bottomsurface 30 of the resin package 3. With this arrangement, it is possibleto surface-mount the capacitor X on a circuit board for example.

[0041] As shown in FIG. 2, the capacitor element 1, which may be a solidelectrolytic capacitor element, includes a capacitor chip 10 and ananode wire 11 projecting therefrom. The capacitor chip 10 comprises asintered porous mass 12 including pores 12 a filled with dielectriclayers 13 and solid electrolyte layers 14. The outer surface of thesintered porous mass 12 is formed with a buffer layer 15 for a cathodeelectrode 16 to be laminated thereon.

[0042] For forming the sintered porous mass 12, metal powder of e.g.tantalum, aluminum or niobium, which works as valve, is compressed intoa cube, and then sintered under high vacuum.

[0043] The dielectric layer 13 may be of an oxidized metal comprised inthe sintered porous mass 12. The dielectric layer 13 maybe formed byperforming anodic oxidation (or by chemical treatment).

[0044] The solid electrolyte layer 14 is formed in the remainig pores ofthe sintered porous mass 12 after the dielectric layer 13 is formed. Thesolid electrolyte layer 14 may be formed by immersing the sinteredporous mass 12 in an aqueous solution of manganese nitrate forimpregnation, followed by a heating treatment. Preferably, the solidelectrolyte layer 14 thoroughly fills the pores 12 a of the sinteredporous mass 12. For this, normally the impregnation and heatingtreatment is performed repetitively.

[0045] The buffer layer 15, which may be made of a graphite, is providedfor reducing the contact resistance between the solid electrolyte layer14 and the cathode electrode 16. For example, the buffer layer 15 isprovided when the solid electrolyte layer 14 is made of MnO₂ while thecathode electrode 16 is of silver. In this way, the buffer layer 15 isprovided as needed.

[0046] As illustrated in FIG. 1, the cathode electrode 16 is connectedto the inner lead portion 20 a of the cathode lead 20 with the aid of aconductive bond 40. The cathode lead 16 may be formed by silver-plating.As for the conductive bond 40, a silver paste or a solder paste may beemployed.

[0047] The anode wire 11 is made of a metal with one end buriedintegrally in the sintered porous mass 12. The anode wire 11 may be madeof the same metal as the one used for forming the sintered porous mass12. The anode wire 11 is connected to the inner lead portion 21 a of theanode lead 21.

[0048] In making the capacitor element 1, the surface of the anode wire11 may be oxidized by the sintering process of a compressed mass or theheating treatment performed for forming the solid electrolyte layer 14.Consequently an oxidized film 11 a is formed on the surface of the anodewire 11. This oxidized film 11 a is partially removed by laserirradiation, as described later, to expose a core conductor 11 b. Thethus obtained exposed portion 11 c helps the anode wire 11 connect tothe inner lead portion 21 a of the anode lead 21.

[0049] The capacitor X may be obtained in the following manner.

[0050] First, as shown in FIG. 3, the anode wire 11 of the capacitorelement 1 is subjected to a laser irradiation (laser irradiation step).The laser irradiation removes the oxidized film 11 a formed on thesurface of the anode wire 11, thereby exposing the core conductor 11 b.The laser irradiation is performed by generating a laser beam with ascan head 5 of a laser oscillator for scanning the anode wire 11longitudinally thereof as indicated by an arrow in FIG. 3. In this way,the exposed portion 11 c is formed longitudinally of the anode wire 11as shown in FIGS. 4a and 4 b. As shown in FIGS. 5a, 5 b, the oxidizedfilm 11 may be removed from two opposite portions across the diameter.In order to save time by an efficient laser irradiation, however, it ispreferable to perform the removal of the oxidized film 11 only with aportion used for connection to the anode lead 21 (FIGS. 4a and 4 b).

[0051] There is no mechanical load (external force) on the anode wire 11during the laser irradiation. In other words, no external force is givento the anode wire 11 unlike the case where the oxidized film is removedby sandblasting or by making a cut in an inner electrode of the anodewire. Consequently, no load is imposed on the portion of the capacitorelement 1 where the anode wire 11 is connected, which avoidsdeterioration in electric characteristics that would otherwise arisefrom the load. In addition, even if the anode wire 11 is downsized inaccordance with the size reduction of the capacitor element 1, the laserirradiation onto the anode wire 11 can still be performed properly byreducing the diameter of the laser spot. In this point, it can be saidthat the present invention provides a technique adaptive for sizereduction of the capacitor element 1 or the capacitor x.

[0052] Various kinds of lasers are known and can be utilized for thelaser irradiating device. A preferable one may be a YAG laser. When theYAG laser is used, the wavelength of the laser beam for irradiating theanode wire 11 is set to be 1000-1400 nm, for example. As in thewavelength of the laser beam, the irradiation period of the laser beamcan be arbitrarily determined as long as the desired objective isachieved.

[0053] The laser irradiation can be utilized for roughening the surfaceof an anode electrode. In this case, the laser irradiation is performed,even if no oxidized film is formed on the surface of the anodeelectrode, or after the removal of the oxidized film is performed.

[0054] Next, the mounting process of the capacitor element 1 isperformed. This process utilizes a lead frame 6 shown in FIG. 6. Thelead frame 6 includes a pair of sidebars 60 and a plurality of crossbars61 bridging between the sidebars 60. The sidebars 60 and the crossbars61 surround regions which are zoned by partition bars 62, and each ofthe zoned regions makes a capacitor mounting zone 63. Each of thecrossbars 61 includes first and second conductor pieces 64, 65 directingtoward the adjacent crossbars 61. As shown in FIG. 7, each of the firstconductor pieces 64 includes an end portion 64 a which is set low.

[0055] As shown in FIG. 8, the mounting process of the capacitor element1 is performed with the use of a conductive bond 70. Specifically, theconductive bond 70 is applied on the end portion 64 a of the firstconductor piece 64, and the capacitor element 1 is mounted thereon so asto locate the capacitor chip 10 on the end portion 64 a, followed bycuring or hardening of the conductive bond 70. In this way, thecapacitor element 1 is mounted on the first conductor piece 64 with thecathode electrode 16 conductive to the first conductor piece 64. At thisstage, as shown in FIGS. 9 and 10, it is preferable that the uncoveredconductor 11 b of the anode wire 11 is in contact with the secondconductor piece 65. For the conductive bond 70, a silver paste or asolder paste is employed for example.

[0056] Subsequently, the connection of the anode wire 11 is performed.As shown in FIGS. 9 and 10, this process step includes resistancewelding performed by employing a pair of electrodes 71, 72.Specifically, the electrodes 71, 72 sandwich the anode wire 11 togetherwith the second conductor piece 65 while the exposed portion 11 c of theanode wire 11 and the second conductor piece 65 are in contact to eachother. Then, a current is passed through the portion where they are incontact.

[0057] Instead of the resistance welding, other methods may be employedfor connecting the anode wire 11. For example, the conductive bondapplied in the mounting of the capacitor element 1 maybe utilized, ornon-electric energy such as heat or ultrasonic wave may be applied forconnecting the anode wire 11 and the second conductor piece 65.

[0058] The anode wire 11, with the oxidized film 11 a removed, isconnected to the second conductor piece 65 by applying energy to theconnecting portion. The absence of the oxidized film ensurescompatibility or facilitates atomic dispersion at the connectingportion. Also, the connection can be made properly with a conductivebond in the presence of a restricted amount of oxidized material betweenthe conductive bond and the inner electrode. Therefore, regardless ofthe method employed, the removal of the oxidized film 11 a from theanode wire 11 serves to strengthen the connection between the anode wire11 and the second conductor piece 65. Regarding the surface rougheningfor the anode wire 11 by the laser irradiation, it increases the surfacearea of the anode wire 11 and improves the wetting characteristicsthereof, thereby contributing to the connection enhancement between theanode wire 11 and the second conductor piece 65.

[0059] The dimensions (thickness, for example) of the oxidized film 11 aformed on the anode wire 11 are irregular for the respective capacitorelements 1. Accordingly, the presence of the oxidized film 11 a on theanode wire 11 causes irregularities in the amount of the current passingthrough the respective second conductor pieces 65 for the innerelectrode connection. As a result, the connecting strength between theanode wire 11 and the second conductor piece 65 varies for therespective capacitor elements 1. The experiment carried out by theinventor of the present invention showed that the strength irregularitybecomes more noticeable as the amount of the oxidized film 11 a left onthe anode wire 11 is smaller. Thus, the removal of the oxidized film 11a is preferably restricted to a small extent, as shown in FIGS. 4a and 4b, for restraining the irregularity of the connecting strength.

[0060] Next, the resin packaging process is performed. In this process,use is made of an upper and a lower molds 73, 74 as shown in FIG. 11.The upper and the lower molds 73, 74 make a cavity 75 when they areclamped. For resin packaging, the capacitor element 1 is placed withinthe cavity 75, which is then filled with a resin poured through a gate76. The resin may be a thermosetting resin such as epoxy resin. Theresin inside the cavity 75 is hardened or solidified, and then the moldsare opened for providing the capacitor element 1 enclosed in the resinpackage 3 as shown in FIG. 12.

[0061] Thereafter, the capacitor X shown in FIG. 1 is obtained bycutting the first and the second conductor pieces 64, 65 and thenbending the conductor pieces (leads 20, 21 in FIG. 1) extending outsidethe resin package 3.

[0062] The inventor of the present invention carried out an experimentfor comparison of samples (sample groups A-C) and other samples (samplegroup D) on the tensile strength at the connecting portion of the anodewire and the lead frame, the former samples being prepared by removingthe oxidized film on the anode wire by the YAG laser for exposure of theconductor surface, which was then connected to the lead frame, thelatter samples being prepared without removing the oxidized film fromthe anode wire before connected to the lead frame. The results are shownin FIG. 13. Each group consists of eleven samples. The tensile strengthis presented in ratio relative to the oxidized film-intact sample groupD whose average is 1.

[0063] The oxidized film was removed from the anode wire in three ways,that is, from the lead frame side only (sample group A), from the leadframe side and the opposite side (sample group B), and from the oppositeside of the lead frame only (sample group C). Resistance welding wasemployed for connecting the anode wire to the lead frame. Weldingconditions (flow amount of the current, period of current flow, usedelectrode) were the same for the respective samples.

[0064] As seen from FIG. 13, the average tensile strength is greater inthe film-removed sample groups A-C than in the film-intact sample groupD. Especially, the average tensile strength ratios of the sample groupsA, B are high. From these results, it is known that the connectionbetween the anode wire and the lead frame is performed well when theoxidized film is removed at least from the lead frame side.

[0065] Further, a variation in the tensile strength is observed in eachof the groups A-D, and the variations are smaller in the film-removedsample groups A-C than in the film-intact sample group D. In particular,the variation of the tensile strength is very small in the sample groupA where the oxidized film is removed from the lead frame side only.

[0066] In summary, the partial removal of the oxidized film using alaser ensures great and stable tensile strength (connecting strength)for connection to the lead frame. For reliable effects, the removal isperformed with respect to the oxidized film on the lead frame side (theconnecting side) only.

[0067] The preferred embodiments of the present invention being thusdescribed, it is obvious that the same may be varied in various ways.Such variations should not be regarded as a departure from the spiritand scope of the invention, and all such variations as would be obviousto those skilled in the art are intended to be included within the scopeof the claims given below.

1. A method for making a capacitor comprising: a capacitor elementincluding a first and a second electrodes; a first lead electricallyconnected to the first electrode; and a second lead electricallyconnected to the second electrode; the method comprising: a laserirradiation step for irradiating the first electrode with a laser beam;and a connection step for connecting the first electrode to the firstlead after the laser irradiation step.
 2. The method according to claim1, wherein the first electrode comprises a metal wire on which anoxidized film is formed, and wherein the laser irradiation in the laserirradiation step is continued until at least a part of the oxidized filmis removed and a metal surface of the wire is exposed.
 3. The methodaccording to claim 2, wherein in the connection step, the exposed metalsurface of the wire is held in contact with the first lead, and energyis applied to the contact portion.
 4. The method according to claim 3,wherein in the laser irradiation step, selective removal is performedexclusively with respect to a portion on the wire that comes intocontact with the first lead.
 5. The method according to claim 3, whereinthe connection step employs resistance welding.
 6. A method for making asolid electrolyte capacitor comprising: a capacitor element including anelement chip, an anode wire projecting from the element chip, and acathode electrode formed on outer surfaces of the element chip; an anodelead electrically connected to the anode wire; a cathode leadelectrically connected to the cathode electrode; and a resin packageenclosing the capacitor element, a portion of the anode lead, and aportion of the cathode lead; wherein the method comprises: a laserirradiation step for irradiating the anode wire with a laser beam; and aconnection step for connecting the anode wire to the anode lead afterthe laser irradiation step.
 7. The method according to claim 6, whereinthe anode wire is formed with an oxidized film, and wherein in the laserirradiation step, the laser irradiation is continued until at least apart of the oxidized film is removed and a metal surface of the wire isexposed.
 8. The method according to claim 7, wherein in the connectionstep, the exposed metal surface of the anode wire is held in contactwith the anode lead, and energy is applied to the contact portion. 9.The method according to claim 8, wherein in the laser irradiation step,selective removal is performed exclusively with respect to a portion onthe wire that comes into contact with the first lead.
 10. The methodaccording to claim 8, wherein the connection step employs resistancewelding.
 11. A method of processing a capacitor element comprising: anelement chip; an anode wire projecting from the element chip andincluding a surface formed with an oxidized film; and a cathodeelectrode formed on an outer surface of the element chip; the methodcomprising a step of: removing at least a part of the oxidized film onthe anode wire by laser irradiation.